Such tubular heating elements may be used wherever a fluid medium is to be heated. This is particularly the case with various household appliances such as washing machines, dishwashers, coffee machines, etc.
In some of these applications, the length of the tubular heating element and the diameter of its heating coil, factors which co-determine its heating capacity, can be freely defined depending on the desired performance features, since the available space for installation for the tubular heating element is not subject to restrictions. Only at the terminal ends of this tubular heating element is it necessary to reduce the heating capacity, because in most cases the medium to be heated circulates only partially or not at all around these areas, with the consequence that the heat produced there is not transported away to a sufficient extent. This means that if the same heating capacity is generated in the area of the terminal ends as in the rest of the tubular heating element, this could result in damage to the heating coil due to overheating. For this reason, connection elements such as terminal pins, etc., are inserted into the inside of the jacket pipe in the terminal portion of the tubular heating element in order to prevent the generation of heat there. This is why the terminal end portions of the tubular heating element are also referred to as the “cold” ends.
The problems outlined above are greatly exacerbated in fields of application in which only little space is available for installing the tubular heating element, but a high heating capacity is still required. In order to achieve good heat transfer to the jacket pipe, it may be necessary to increase the diameter of the heating coil accordingly. However, this results in a high heating capacity likewise being present at the terminal ends, which can lead to the problems already outlined in the foregoing. In tubular heating elements of this kind, that is to say in tubular heating elements with a short overall length and large heating capacity, what can also be done in many cases is that the tubular heating element is bent at its terminal ends in order to have an easy connection to a voltage source. When the heating element is bent in this way, the heating coil inside the jacket pipe of the tubular heating element is also bent along its longitudinal axis, which leads to undefined deformation of the heating coil. The spiral coil may also be displaced in parts, leading to locally overheated areas due to uneven transfer of heat.
The terminal end portion of a known tubular heating element is shown in FIG. 4. This tubular heating element has a heating coil 22′ embedded in an electrically insulating material 20′ inside a jacket pipe 10′. Heating coil 22′ is connected on this side to a source of current by means of a terminal pin 24′. To prevent the separate spirals of heating coil 22′ from taking up an undefined position in the bent or curved portion 18′, the terminal pin is guided beyond the curved portion of jacket pipe 10′ into the inside of the tubular heating element, where the heating coil 22′ is finally connected to terminal pin 24′. However, the heating capacity is further reduced as a result, because appropriate heating capacity is not present until heating coil portion 22′. The diameter of heating coil 22′ is also adapted to the diameter of terminal pin 24′. Since the diameter of pin 24′ is small compared to the inner diameter of jacket pipe 10′, only a small heating capacity can be produced when the jacket pipe has such a maximum diameter.